Method for corrosion protection



A. ALMAR-NAEss 3,350,288

METHOD FOR CORROSION PROTECTION Oct. 31, 1967 Filed April 15, 1964 2Sheets-Sheet 1 INVENTOR. ALMAR ALH AR- Mess BY y MM AGEA United StatesPatent Office 3,350,288 METHOD FOR CORROSION PROTECTION AlmarAlmar-Naess, Frydenbergveien 12,

- Trondheim, Norway Filed Apr. 15, 1964, Ser. No. 360,000 Claimspriority, application Norway, Apr. 27, 1963,

3 Claims. (Cl. 204-147) ABSTRACT OF THE DISCLOSURE Cathodic protectionand cleaning of a drying cylinder of a paper-making machine is achievedby applying a negative charge to the cylinder, fixedly arranging anelectrode adjacent a portion of the surface of the cylinder which is notcovered by the paper web being dried on the cylinder, applying apositive charge to this electrode, pressing an electrolyte through thegap between the electrode and the cylinder surface, and passing acurrent between the electrode and this cylinder surface.

The present invention relates to a method for corrosion protection andcleaning of a drying roller of a paper-making machine.

It is known that a high degree of protection of a metal'or a metal alloyagainst corrosion may be achieved by making the metal electricallynegative in relation to the surrounding electrolyte. This is theprinciple of cathodic corrosion protection. The potential difference maybe obtained e.g. by an outside source of potential, or by a more or lesspermanent anode, such as for example when a buried pipe is protected. Itis usually assumed that the magnitude of the potential differencebetween the metal and the adjacent liquid layer determines the degree ofprotection obtained. A uniform protection of the surface, therefore,means a uniform potential difference, which again means a uniformcurrent density on the surface.

A uniform current density on the surface is not always obtainable. Thecurrent density is, inter alia, dependent upon the placing of the anode.In the cathodic protection of pipes, as mentioned, the anodes may bedistributed in any desired manner and there is no difiiculty inobtaining a uniform current density at least as long as the conductivityof the soil is reasonably uniform. If, however, the outside of a ship isto be protected, the anodes are usually distributed along the side ofthe ship, whereby the current density close to the anode becomes muchgreater than that further away from it.

In paper making, a wet paper web is brought into contact with thesurface of the drying roller at one point and leaves the roller surfaceat another point remote from the one point, leaving an uncovered rollersurface between the one and the other point. The surface of the dryingroller is cathodically protected against corrosion and cleaned inaccordance with the invention by electrically connecting the dryingroller to a negative pole of a direct current source, arranging anelectrode in a fixed position with respect to the axis of the dryingroller and adjacent a portion of the uncovered roller surface, a gapbeing defined between the electrode and the uncovered roller surfaceportion, pressing an electrolyte through this gap, electricallyconnecting the electrode to a positive pole of the direct currentsource, and passing a current between the electrode and the uncoveredroller surface portion whereby hydrogen gas is evolved on the surface ofthe drying roller.

The invention is described with reference to the accompanying drawing inwhich,

3,358,288 Patented Oct. 31, 1967 FIGURE 1 is a schematical verticalsection of a drying drum for crystallizing solids,

FIGURE 2 is a schematical vertical section of a Yankee cylinder of apaper-making machine,

FIGURE 3 is a vertical section on a larger scale of the anodearrangement 14 of FIGURE 2, and

FIGURE 4 shows diagrammatically the relationship between the electriccurrent and the overvoltage obtained and employed according to theinvention.

A rotating drum which is partly in contact with a liquid, as shownschematically in FIGURE 1, can not be protected readily according to theknown cathodic principle, as a current can be passed only through thatpart of the drum which is submerged in the liquid.

FIGURE 1 shows a drum 1 for crystallizing solid materials from asolution 2. The drum is Warm, the liquid evaporates from the surface 3,and the solids 5 are scraped off by the drum by scrapers or doctorknives 4.

A corresponding problem arises if it is desired to employ cathodicprotection for cylinders for drying paper. FIGURE 2 shows how paper web12 and felt backing 11 at one point is brought into contact with, and atanother point remote from the one point leaves, the surface of a Yankeecylinder 10 which may be considered to be a very simple paper-makingmachine. The paper has a moisture content of about 50% on contacting thecylinder, and the cylinder will be corroded on that area where the paperis wettest. The rate of corrosion depends, inter alia, upon the saltcontent of the water.

In the method according to the invention, an anode 17 is arranged eitherin the area where the paper and felt come on to the cylinder in a wetcondition, e.g. between the pressure rolls 16 (FIG. 2), or preferably onthe dry area of the cylinder, e.g. at 14 between two of the doctorknives 13 designed to remove remnants of paper and possible corrosionproducts from the cylinder.

As known, the electrolytical double layer between a metal surface andthe adjacent electrolyte may be considered a condenser C connected inparallel to a nonlinear resistance Rv, as shown in FIG. 4 e, in thediagram denotes a constant voltage simulating the equilibrium potentialbetween the metal concerned and the electrolyte, and 1 is theovervoltage. n is defined approximately by Tafels equation:

E dn+10 1 (1 La b C z 10 According to this equation the overvoltage willvary principally as shown in FIG. 4 on switching the current on, andoff. It will be seen that the overvoltage remains on the surface forsome time after the current has been interrupted, which again means thatthe metal surface is protected against corrosion for some time after theinterruption of the current. This after-effect period is in the firstinstance dependent upon the employed current density, and also, asappears from the equation above, upon the constants a and b, and thecapacity C of the double layers. Measured values of these quantitiesshow that in a neutral electrolyte such as 0.21% NaCl in water,corrosion resistance is obtained for about 60 seconds after the currentinterruption at a current density of about 0.1 amp./cm.

When, an anode 17, e.g. of platinum, is arranged on cylinder 10,pressing against the felt 11 between the two pressure rolls 16, so thatthe applied current passes the wet felt and wet paper, the wholecylinder will be protected against corrosion. The current density mustbe adjusted to the cylinder speed, the cylinder material and the actingelectrolyte. A corresponding corrosion protection is also achieved ifthe anode 14 is placed on the dry area of the cylinder between the twoknives 13, which is also shown in FIG. 2. A useful anode construction isindicated in FIG. 3. The block 21 is made of some electricallyinsulating material, 22 is the anode. This may consist of platinum,graphite, lead, or other material which is fairly resistant to an anodicload. Between the anode 22 and the cylinder 25 water 23, 24 havingsufiicient electrical conductivity, is passed, so that current may passbetween them and establish an electrolytic double layer on the surfaceof the cylinder. If the cylinder is warm, the water will no doubtevaporate therefrom, so that it is dry outside the block 21. It has,however, been found that an overvoltage may still be measured on thecylinder surface at the place where the wet paper and felt come on tothe cylinder, or at any other place where the cylinder is moist and apotential measurement is possible. The reason for this fact is at thepresent not entirely clear. As the double layer is connected with anelectrolyte, it is difficult to realize how it can continue to existwhen the electrolyte has evaporated. It may be guessed that theapparently dry cylinder surface is still covered by a layer of watermolecules. It may also be assumed that the double layer is formedbetween the metal and a thin layer of metal oxide, or that an ionmigration takes place in the oxide layer so that the equilibriumpotential is changed. The invention is, however, not limited to anyspecific theory.

If the cylinder is contaminated by an oxide layer, this will, undercertain conditions, be dissolved by the cathodic charge. A cylinder ofsteel or cast iron, for example will normally be coated by a layer ofrust wherein the iron is present in the trivalent state, and, therefore,is insoluble in water. By a cathodic charge the trivalent iron isreduced to the divalent state wherein it is soluble in the anode waterand it is consequently removed with this. After some time of operation,such a cylinder will, therefore, be cleaned of rust.

The primary electrochemical reaction on the cylinder is hydrogenevolution. The hydrogen loosens burnt-on paper remnants from thecylinder, and they are carried away with the anode water. This action isof great importance for a clean cylinder surface, as the fiber and paperremnants can not always be removed by the ordinary scrapers.

If a suitable metal electrolyte is used instead of water between thecylinder and anode, the cylinder may be coated electrolytically with ametal layer which is more resistant to corrosion than the base material.The coating may be made very thin, it may be applied during operation ofthe cylinder, and possible wear, peeling, and the like, may be repairedduring operation.

By making the cylinder 25 the anode in the circuit shown in FIG. 3, andby employing a suitable electrolyte and current density, it is possibleto achieve an electrolytic polishing of the cylinder surface. Byadjusting the electrolyte and the current density the cylinder-may inthis way also be protected anodically against corrosion, i.e. thesurface is passivated.

It will be appreciated that the foregoing description is onlyillustrative of the present invention and it is intended that hisinvention be limited only by the hereinafter appended claims.

I claim:

1. An apparatus for corrosion protection and cleaning of the surface ofa cylindrical metal drying roller of a paper-making machine, comprising(a) the roller,

(b) a container open on one side,

(1) the container being arranged water-tightly against the surface ofsaid roller, with the open container side facing the surface,

(c) an electrode in said container and spaced from the surface,

(d) means for passing an electrolyte into and out of the container; and

(e) means for passing a direct current between the electrode as theanode and the roller as the cathode.

2. A method for the cathodic corrosion protection and cleaning of ametallic drying roller of a paper-making machine, wherein a wet paperweb is brought into contact with the surface of the drying roller at onepoint and leaves the roller surface at another point remote from the onepoint, leaving an uncovered roller surface between the one and the otherpoint, the steps of (l) arranging an electrode in a fixed position withrespect to the axis of the drying roller and adjacent a portion of theuncovered roller surface,

(a) a gap being defined between the electrode and the uncovered rollersurface portion,

(2) passing an electrolyte through said gap,

(3) passing a direct current between said electrode as the anode andsaid uncovered roller surface portion as the cathode.

3. The method of claim 2, further comprising the steps of confining saidelectrolyte in a chamber, one wall of said chamber being the uncoveredroller surface portion and another chamber wall being said electrode,and maintaining a water-tight seal between said chamber and saiduncovered roller surface portion.

References Cited UNITED STATES PATENTS 1,746,964 2/1930 Polatsik 204-1492,847,375 8/1958 Murphy 204197 3,025,790 3/1962 Larsen 10l149.23,063,921 11/1962 Leibowitz 2041 10 3,166,487 1/1965 Owren 2041963,193,450 7/1965 Herbert et al. 162-192 3,196,101 7/1965 Hosford 204l963,201,335 8/1965 MacNab et al. 204147 3,231,480 1/1966 Collins 2041473,242,064 3/1966 Byrne 204196 3,254,012 5/1966 Ziegler 2O4147 FOREIGNPATENTS 449,079 9/1927 Germany.

JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

2. A METHOD FOR THE CATHODIC CORROSION PROTECTION AND CLEANING OF AMETALLIC DRYING ROLLER OF A PAPER-MAKING MACHINE, WHEREIN A WET PAPERWEB IS BROUGHT INTO CONTACT WITHD THE SURFACE OF THE DRYING ROLLER ATONE POINT AND LEAVES THE ROLLER SURFACE AT ANOTHER POINT REMOTE FROM THEONE POINT, LEAVING AN UNCOVERED ROLLER SURFACE BETWEEN THE ONE END ANDTHE OTHER POINT, THE STEPS OF (1) ARRANGING AN ELECTRODE IN A FIXEDPOSITION WITH RESPECT TO THE AXIS OF THE DRYING ROLLER AND ADJACENT APORTION OF THE UNCOVERED ROLLER SURFACE, (A) A GAP BEING DEFINED BETWEENTHE ELECTRODE AND THE UNCOVERED ROLLER SURFACE PORTION, (2) PASSING ANELECTROLYTE THROUGH SAID GAP, (3) PASSING A DIRECT CURRENT BETWEEN SAIDELECTRODE AS THE ANODE AND SAID UNCOVERED ROLLER SURFACE PORTION AS THECATHODE.